Pretty much everyone knows what a half sibling is – someone who shares one, but not both parents with each other.
If you and your sibling share the same mother, but not the same father (or vice versa), you’re half-siblings. Only one parent is shared between half-siblings.
If you share both parents, you’re full siblings.
Step-Siblings
Step-siblings are often confused and used interchangably with half-siblings, but they aren’t at all the same. And yes, it matters.
If your parent married someone who already had a child, but both you and that child were born to prior (or future) marriages/relationships of your respective parent – you’re step siblings.
In other words, your parents are married to each other, and you may live in the same household, but you don’t share a genetic link with a step-sibling because you don’t share any parent in common.
By way of example, my mom married a man who had a son from a prior marriage, and his son is my stepbrother. The man my mother married is my stepfather.
My mother is my stepbrother’s stepmother.
The real message here, other than clarifying confusing relationship terms that are often used incorrectly, is that:
- There is a biological relationship between full-siblings and half-siblings
- There is no biological relationship between step-siblings, unless one exists due to ancestors someplace in the past
I wrote about how much of your ancestors’ DNA you can expect to inherit in the article, Ancestral DNA Percentages – How Much of Them is in You?.
Genetically, Half Versus Full Matters
The amount of autosomal DNA that is expected to be shared between full-siblings and half-siblings differs. Throughout this section, I’m using words like “expected” and “about” because in reality, after parents, “exactly” half of the ancestral DNA in a specific generation does not get passed to the next generation. Random recombination is a factor and therefore, the expected inherited percentages are approximate.
Full siblings share both parents, while half-siblings share only one parent, so full siblings share about 50% of their DNA, while half-siblings share about 25% of their DNA – and only from one parent.
Therefore, every descendant relationship from full or half relationships varies by 50% between the two types of relatoinships.
Half-siblings can be expected to share, on average, half as much autosomal DNA as full siblings – because they only share one parent – not two.
For example, first cousins (1C) share about 12.5% of their DNA, but half first cousins (half 1C) share about 6.25% of their DNA.
In other words, the “half” designation literally means that those two people share half a relationship – one parent (or grandparent, etc.), not both, in the founding generation, and their descendants continue to share half as much DNA as a full relationship in the same generation.
Subsequent Generations
Extending those relationships down the tree generation by generation, we see that in each subsequent generation, the descendants can be expected, on average, to share one-fourth as much DNA with each other as the preceding generation. That’s because two transmission events have taken place, one in Child 1’s line, and one in Child 2’s line.
The same as in full sibling lineages, each subsequent half-sibling descendant generation can also be expected to share one quarter as much autosomal DNA as the preceding one.
“Removed” Relationships
If you encounter a situation where one side of the descendant tree is “longer” than the other by a generation or more, then you’re dealing with a phenomenon known as “removed,” such as first-cousin-once-removed (1C1R), or, in the example above, a third cousin (3C) once removed (1R).
The same concepts still apply. A half 3C1R would share half as much DNA as a 3C1R.
I wrote all about “removed” relationships and their genetic genealogy effects in the article Concepts: What Does a Cousin “Once Removed” Mean.
Summary
In this final chart, I’ve combined the full-sibling and half-sibling charts into one so that you can compare them easily. I’ve also removed the “other” parent that the half-siblings don’t share to conserve space.
I wrote about how much DNA each type of relationship can be expectd to share, both centiMorgans (cMs) and percentages, including relationships not detailed here, such as half-uncles and half-aunts, in the article Shared cM Project 2020 Analysis, Comparison & Handy Reference Charts.
_____________________________________________________________
Share the Love!
You’re always welcome to forward articles or links to friends and share on social media.
Subscribe!
If you haven’t already subscribed (it’s free,) you can receive an e-mail whenever I publish by clicking the “follow” button on the main blog page, here.
Help Keep This Blog Free
I receive a small contribution when you click on some of the links to vendors in my articles. This does NOT increase your price but helps me keep the lights on and this informational blog free for everyone. Please click on the affiliate links in the articles or to the vendors below if you are purchasing products or DNA testing.
Thank you so much.
DNA Purchases and Free Uploads
- FamilyTreeDNA – Y-DNA, mitochondrial and autosomal DNA testing
- MyHeritage DNA – Autosomal DNA test
- AncestryDNA – Autosomal DNA test
- AncestryDNA Plus Traits
- 23andMe Ancestry – Autosomal DNA only, no Health
- 23andMe Ancestry Plus Health
Genealogy Products and Services
- MyHeritage Subscription with Free Trial
- Legacy Family Tree Webinars – Genealogy and DNA classes, subscription-based, some free
- Legacy Family Tree Software – Genealogy software for your computer
- OldNews – Old Newspapers with links to save to MyHeritage trees
- MyHeritage Omni comprehensive “everything included” subscription plan
- Newspapers.com – Search newspapers for your ancestors
- NewspaperArchive – Search different newspapers for your ancestors
My Books
- DNA for Native American Genealogy – by Roberta Estes, for those ordering the e-book from anyplace, or paperback within the United States
- DNA for Native American Genealogy – for those ordering the paperback outside the US
- The Complete Guide to FamilyTreeDNA – Y-DNA, Mitochondrial, Autosomal and X-DNA – for those ordering the e-book from anyplace, or paperback within the United States
- The Complete Guide to FamilyTreeDNA – Y-DNA, Mitochondrial, Autosomal and X-DNA for those ordering the paperback from outside the US
Genealogy Books
- Genealogical.com – Lots of wonderful genealogy research books
- American Ancestors – Wonderful selection of genealogy books
Genealogy Research
- Legacy Tree Genealogists – Professional genealogy research
Share this:
Discover more from DNAeXplained - Genetic Genealogy
Subscribe to get the latest posts sent to your email.
Thank you so much Roberta. You always explain so concisely and in a way I can understand. It has enabled me to discover that my father’s relationship to his siblings?? was questionable.
Thank you. If it’s still questionable, check to see if the DNA matches the ancestors of both parents if the right people are still available to test.
No one still alive. But his sister?? was 18yrs when my father was born and I have DNA matches on her maternal line but none on her paternal line. I’m currently sifting through my brother’s PATERNAL DNA MATCHES with little success. Am I on the right track. Think I need some professional help.
As an identical twin we know our children are the same as half siblings, our husbands are not even related. One of my sons shares more DNA with my sister than her own daughter. That is just an interesting fact along with “Therefore, every descendant relationship from full or half relationships varies by 50% between the two types of relatoinships.”
She did Ancestry DNA and I did 23&Me to see how they differ, and it isn’t all that much. We also uploaded to Gedmatch to see we are
99.983 full identical and speculate the slight difference is the fact we’ve both had children, she has two daughters, I have two sons and one daughter.